Frequency translating system



APril 1932- c. R. KEITH 1,855,576

FREQUENCY TRANSLATING SYSTEM Filed April 9, 1929 Hal 67/85 l/NE sun u/v: Hp

INVENTDR CE! KEITH Arm Patented ,Apr. 26, 1932 UNITED STATES PATENT OFFICE CLYDE B. KEITH, OF NEW YORK, N. Y., ASSIGNOR TO BELL TELEPHONE LABORATORIES, INCORPORATED, 01 NEW YORK, N. Y., A CORPORATION OF NEW YORK FREQUENCY TRANSLATING SYSTEM Application filed April 9, 1929. Serial No. 853,840.

This invention relates to frequency translation of electric waves and particularly to a system which may be operated to effectmodulation and detection.

An object of the invention is to effect modulation and detection in a system including rectifiers, and particularly copper oxide rectifiers.

Another object is to prevent undesired currents, either impressed upon or developed by the normal operation of the modulator, from being transmitted to circuits external to but associated with thegnodulator.

In accordance with the present invention the modulating. system comprises at least four rectifying elements arranged in pairs and having each pair connected in push-pull or balanced relation. The push-pull pairs are interconnected in such manner as to provide a system having but one pair of terminals, at each side of the system, for connection to an external circuit. A source of carrier waves is associated with the modulator.

Since the rectifying-modulating system has only two pairs of binding posts or terminals through which connection is made to external circuits at the respective sides there of, it is possible to utilize the system for combining a carrier wave with a modulating current supplied by a source over the external circuit connected to one pair ofterminalsand to detect a modulated wave supplied by another'source over the external circuit connected to the other pair of terminals, without requiring the use of switching means for changing over from one function to the other.

The use of a double balanced circuit tends to prevent waves or currents impressed upon the rectifiers through one of the external circuits from being transmitted to the other external circuit and the other wave source. It also permits the elements of the circuit to be so connected as to balance out certain distorting currents, produced by the normal operation of the rectifiers, which would tend to flow through the external circuits and the wave source.

The invention will be more fully described hereinafter with reference to the drawings in which Fig. 1 shows a. double balanced system for efi'ecting modulation and detection in accordance with the invention, and Figs. 2 and 3 show portions of a carrier wave multiplex system embodying a combined modulator and detector.

The system shown in Fig. 1 employs four rectifying units or wave detecting devices 1, 2, 3 and 4. Any device which has a different degree of conductivity for currents in the two directions therethrough may be used. A device which has high conductivity inone direction and practically none in the other is preferable. For this purpose a rectifier composed of one or more discs of copper, each disc coated with copper oxide on one surface and separated from adjacent discs by lead spacers, has proved to be well suited. To facilitate balancing the system, it is generally convenient to use four substantially identical rectifiers.

Three pairs of terminals, 56, 78 and 9-1O are shown, although only two pairs are intended for connection to external circuits. The third pair is provided for connection to a source supplying carrier waves to the modulator-detector.

For reasons which will be set forth hereinafter and when the circuit is used for the purpose of combining a carrier wave with a modulating current to produce side-bands, it is often preferable to connect the carrier source to one of the terminal pairs 7-8 or 910. The second of these pairs, 910 or 7.8, is connected to a load circuit, transmission line or antenna system adapted for the transmission of a modulated carrier wave. The remaining pair 5-6 is connected to a source of signaling or modulating current.

For purposes of disclosing one embodiment of the invention, the circuit of Fig. 1 will be described on the basis that it is to be used for the purpose of combining a signal or modulating current with a carrier wave to produce a signal modulated wave and on the assumption that the carrier source is connected to terminals 7-8, that the modulated wave is to'be supplied to an external circuit through terminals 910, and that the source of modulating current is connected to terminals 56.

The various airs of terminals are connected to the rectifying devices by means of the transformers 11, 12, 13 and 14 and the current loads 26 and 27 The transformer 11 includes a rimary windin 15 and a secondary win ing 16. The en s of winding- 15 are connected to the terminals 5 and 6 respectively.

Transformer 12 includes a balanced air of windings 17 and 18, and a third win 'ng 19. Transformer 13 includes a balanced (plzir of windings 21 and 22, and a third win g 20. These windings are also balanced in the additional pairs 17-22, 18-21, 17-21, 18-22, and 19-20. It is enerally convenie'nt and conducive. to com ete balancing to make transformers 12 an 13 substantially identical.

Transformer 14 includes a pair of balanced windings 23-24 and a third winding 25, the terminals of the latter winding being connected to terminals 9 and 10.

Rectifying-devices 1 and 2, conductivelyopposed, are connected in series with the windings 17 and 18. Rectifying devices 3 and 4, also conductively in opposition, are connected in series with windings 21 and 22;

The two ends of the windin 16 are connected res tively to the junction 28 of devices 1 an 2 and the junction 29 of devices .3 and 4. Lead 26 provides a direct connection between the junction 28 and the common terminal 30 of windings 21 and 22. Lead 27 provides a direct connection between the unction 29 and the common terminal 31 of windings 17 and 18.

The windings 19, 20, 23 and 24, connected in series, comprise a coupling circuit generally designated hereinafter by reference numeral 40. Terminal 7 is connected to the junction 32 of windin s 19 and 20 and terminal 8 is connected to t e junction 33 of windings 23 and 24.

The system shown in Fig. l is so balanced that the sources of modulating current and carrier wave, connected respectively to the terminal pairs 5-6 and 7-8, are substantiall prevented from transmitting current of thelr respective frequencies into circuits connected to the other pairs of terminals.

For example, modulating currents from the source connected to terminal pair 5-6, flowing in winding 15 will, during one-half cycle, set up a current upwardly through windin 16 into lead 26 and returning to winding16 ly two parallel paths comprlsing respective y, win ing 21 in series with rectifier 3 and winding 22 in series with rectifier 4. Since windings 21 and 22 are balanced, no appreciable current is induced in circuit 40 durin this half cycle.

uring the remaining half cycle a current is set up in a direction downwardly through winding 16 into lead27 and returnin to winding 16 by two parallel paths compr' mg,

respectively, winding .17 in series with rectifier 1 and winding 18 in series with rectifier 2. Windings 17 and 18 bein balanced, no appreciable current is induced in circuit 40 during the second half cycle.

Since no ap reciable currents are set up in circuit 40 during either of the half cycles of the modulatin current, no appreciable amount of mofulating current is transmitted to either pair of terminals 7-8 or 9-10 associated with that circuit.

On the other hand, waves supplied by the source through terminals 7-8 cause carrier currents to ow through two parallel paths comprising, respectively, windings 19 and 24 in series and windings 20 and 23 in series. Due to the fact that windings 23 and 24 are balanced, no appreciable carrier current is .transmitted to the terminal air 9-10.

. Furthermore, during onealf cycle of the carrier wave, the carrier currents in windings 19 and 20 set up electromotive forces in windings 17 and 22 in such directions as to aid eac other inthe circuit through win g '17, rectifier 1, lead 26, winding 22, rectifier 4 and lead27 back to winding 17. B virtue of the balance between winding-s 1 and 22, there is no diflerence of potential at carrier frequency between the leads 26 and 27, so that no ap reciable carrier current flow is roduced in winding 16, which is connecte across the leads 26 and 27. Therefore, no appreciable current is induced in winding 15 and transmitted to the terminal pair 5-6.

Similarly, during the remaining half cycle of the carrier wave, the carrier currents in windings 19 and 20 set up electromotive forces in windings 18 and 21 in such directions as to aid each other in producing current flow in the circuit through winding 18, rectifier 2, lead 26, winding 21, rectifier 3 and lead 27 back to winding 18. Due to the balance between windings 18 and 21, the carrier frequency potential of the leads 26 and 27 continue to remain equal to each other and therefore no appreciable carrier current flow is produced in winding 16 and transmitted to terminal pair 5-6.

Since no appreciable currents are transmitted to either pair of terminals 5-6 or 9-10 during either of the half cycles of the carrier wave, it follows that no appreciable carrier currents are directly transmitted from the source, connected to terminals 7-8, to either of the other pairs of terminals.

Currents from the carrier and modulating sources are, however, transmitted through the paths traced above to-th'e rectifying devices 1 to 4 inclusive, wherein they are combined to roduce a modulated wave.

The modulated wave comprises components in various phases. Certain of these components transmitted through transformers 12 and 13 and circuit 40 to the windings producing a current flow in.

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23 and 24, are in such phase relationship in these windings that they additively affect the winding 25. However, other components which tend to produce distortion are balanced out in transformers 12 and 13 or, if transmitted to circuit 10, are opposed to one another in windings 23 and 2 1 and do not induce appreciable current flow n winding 25.

In a similar manner, certain of the current component-s produced by the rectifying devices and flowing through the windings of the transformers 11, 12'and 13, are in such phase relationships that they are additively effective in the circuits associated with terminals 56, 7-8 'and 910, while certain other components, which cause distortion, are subtantially prevented from inducing currents therein.

While in describing the operation of the system of Fig. 1, it has been assumed that the carrier wave source is connected to terminals 7 8, the source of modulating current to terminals 56, and the load circuit to terminals 9-10, it is not necessary that the connections be made as assumed.

In general, the carrier Wave source may be connected to any desired pair of terminals, 56, 78, or 9-10, the source of modulating current may be connected to either of the remaining two pairs, and the load circuit may then be connected to the third pair of terminals.

In many practical cases the carrier wave and the'side bands produced by modulation lie relatively close together in respect to frequency. In such cases, the modulating current generally comprises components having frequencies comparatively remote from the frequency range of the carrier and side bands.

It is often advantageous, therefore, to connect the carrier source and the load circuit to which the modulated wave is to be supplied to the terminals 78 and 910, as assumed above. The transformers 12 and 13, which 1 are employed to connect the carrier and side band circuits to the rectifiers, may then be designed expressly for use in the frequency range occupied by the carrier and side bands. There is then no necessity for any transformer in the system to transmit efiiciently current components lying in bands separated by a relatively Wide frequency interval.

In the example discussed above, the terminals 5 and 6 are connected to the source of modulating current and transformer 11 is designed solely for use in the frequency range of the modulating current.

While the system of Fig. 1 has been described as a modulating system it is obvious that the same system may be employed as a detector or demodulator, since the process of detection or demodulation does not differ essentially from the process of modulation, as is well known.

To eflect demodulation in the system of Fig. 1, a source of modulated waves is applied to one pair of terminals and the earrier source is applied to another pair. The demodulated or detected currents are then supplied, through the third'pair of terminals to a load circuit, transmission line, or the like.

Preferably, the carrier source is connected either to pair 78 or pair910. The second pair, whether 9-10 or 78, is connected to the source of modulated waves. Theremaining pair 56 is connected to the load circuit.

Assuming the source of modulated waves to be connected to the pair 9-10, modulated waves are transmitted to. circuit 40 from this source by transformer 14. On account of the balance existing between windings 19 and 20 and also between windings 23 and 24, the modulated waves will cause equal potentials to be applied to the terminals 7 and 8. The source is, therefore, prevented from producing an appreciable current flow in a circuit connected to the terminals 78.

The source of modulated waves is likewise prevented from producing appreciable current flow in a circuit connected to terminals 5 and 6, by virtue of the balance between certain windings of transformers 12 and 13.

During one-half cycle of the modulated Wave, the modulated currents in windings 19 and 20 set up electromotive forces in windings 17 and 21 in such directions as to aid each other in producing a' current flow in the circuit through winding 17, rectifier 1, lead 26, windlng 21, rectifier 3; and lead 27 back i to winding 17. By virtue of the balance between windlngs 17 and 21, potentials developed in the leads 26 and 27 remain equal to each other, so that no appreciable current flow is produced in winding 16, bridged across leads 26 and 27. Therefore no appreciable current is induced in winding 15 and transmitted to circuit connected to terminals 56.

During the remaining half cycle of the modulated wave, the'modulated currents in windings 19 and 20 set up electromotive forces in windings 18 and 22 in such directions as to aid each other in producing current flow in the circuit through winding 18, rectifier 2, lead 26, winding 22, rectifier 4 and lead 27 back to winding 18. Due to balance between windings 18 and 22, potential applied to the leads 26 and 27 continue to remain equal to each other and, therefore, no appreciable current flow is produced in wind ing 16 and transmitted through transformer 11 to terminals 56.

Since no appreciable currents are transmitted to either pair of terminals 56 or 78 during either of the half cycles of the modulated wave, it follows that no appreciable modulated currents are directly transmitted from the source, connected to terminal pair 9-10, to either of the other pairs of terminals. It follows generally from the above description that a source of current connected to any one of the three pairspf 5-6, -8 or 910, is substantially shielded from currents transmitted from a source connected to one of the remaining pairs of terminals.

Also, currents supplied by a source connected to any one o the terminal lrs will induce currents which are supphed to at least two of the rectifyin devices.

Furthermore, no wave lters are necessary 1 to prevent direct transmission of currents from one source into a circuit connected to another source of current.

Fig. 2 represents diagrammatically a portion of one terminal of a carrier wave multi- Q plex transmission system, showing means for connecting two lines, for example,,t wo telephone subscribers lines, to a main line suitable for high frequencytransmission. M-D

is a combined modulator and detector of the type shown in Fig. 1. The subscribers line L constitutes a source of modulatin current. R is a two-way repeater or ampli er which constitutes a means for transmitting a modulated carrier wave.

The three pairs of terminals of the modulator-detector are connected respectively to the subscribers line L, the carrier wave generator C and the re aterR, either in the pre-v ferred order hereinb fore recommended or in any other desired order. The repeater R is also connected through a band pass filter FBP to the high frequency line.

A second subscribers line L is connected to the same hi h frequency line through a low pass filter LP.

The system of Fig. 2 may be operated either supplied by gento modulate a carrier wave, erator C in accordance with rent from the subscribers line L, to produce a modulated carrier wave. which is repeated by R and transmitted through filter FBP to the high frequency line, or it may be used to receive from the high frequency line a modulated wave which is selectively transmitted through the filter FBP, amplified in repeater R and detected in the modulator detector M-D to produce a signal current which is supplied to the line L, In accordance with well known principles of multiplex practice, the subscribers line L, may be used at the same time to transmit telephone or telegraph signals without employing a carrier wave.

Fig. 3 shows a portion of a multiplex system embod ing a particular form of twoway amplifizar or repeater which may be used if place of the repeater R in the system of With the exception of the amplifier, Fig. 3 is identical with Fig. 2, and for that reason only the amplifier will now be described.

terminals a signaling cur- The amplifying system of Fi 3 comprises two re ating elements R and associated with e hybrid coils H and H, The h brid coils are balanced by means of networ N1 and N3- The am lifier R is employed to amplify a modulated wave produced by the modulatordetector M-D, while the am lifier Rfis employed for amplifyin modu ated waves incoming from the high requency line.

As is well known in the art, one amplifier, for example R may serve to amplify all the modulated waves, whether incomin or outgoing. In this case, the output lea s of amplifier R are connected to the hybrid coil H in (place of the output leads of amplifier'R an the associated network N is re laced by direct connections to the filter This may be done by throwing all of the doublepole, double-throw keys or switches S S S 8,, out of engagement with their respective inner pairs of contacts and into en gement with their res ctive outer pairs ofc ontacts. Amplifier hybrid coil H, and network N are thereby disconnected.

Branch circuits B, B are provided in the systems of Figs. 2 and 3 for connecting the high frequency line to additional carrier wave communication channels (not shown) which may be operated, if desired, in conjunction with the apparatus which has been described.

What is claimed is: 1. A frequency translating system 00mfprising at least four rectifying elements, said elements being balanced in pairs and one of said pairs being associated with another of said pairs in balanced relation, a plurality of circuits for supplying said rectifying elements with currents to be translated and a load circuit for utilizing translated currents; said current supplying circuits and said load circuit being connected respectively to said rectifying elements and said balanced arrangement being eii'ect-ive to prevent direct transmission between any two of said aforementioned circuits.

2. A frequency translating system-comprising at least four substantially unidirectional conductors, said conductors being arranged in pairs and in conductive opposition to each other in each pair, one of said pairs being associated with another of said pairs in balanced relation, a plurality of circuits for supplying said unidirectional conductors with currents to be translated and a load circuit for utilizing translated currents; said current supplying circuits and said load circuit being connected respectively to said unidirectional'conductors and said balanced arrangement being e'flective to prevent direct transmission betweenany two of said aforementioned circuits.

' 3. A combined modulating and detecting system comprising a source of carrier waves,

a load circuit including an associated source of modulating current, a second load circuit including an associated source of modulated waves to be detected, at least four rectifyin elements, means connecting each of said loa circuits and each of said sources respectively with all of said rectifying elements whereby said elements are effective to cause modulation of said carrier waves and detection of said modulated waves,-said connecting means being balanced to prevent direct transmission between any two of said sources.

4. A combined modulating and detecting system comprising a source of carrier waves, a load circuit including an associated source of modulating current, a second load circuit including an associated source of modulated waves to be detected, at least four rectifying elements, means connecting each of said load circuits and each of said sources with all of said rectifying elements, whereby said elements are effective to cause modulation of said carrier waves, and detection of said modulated waves, with incidental generation of distorting currents, said connecting means being balanced to prevent direct transmission between any two of said sources and to reduce the transmission of said distorting currents into said load circuits.

5. A terminal circuit for a high frequency signaling system comprising a Subscribers line, a high frequency line, rier waves and a combined modulator detector including at least four rectifying elements connected in balanced arrangement whereby direct transmission between said lines and between said source and each of said lines is prevented.

In witness whereof, I hereunto subscribe my name this 4th day of April, 1929.

CLYDE R. KEITH.

a source of car-' 

